In general aviation, according to established aerodynamic stability and control law, an aircraft should be stable in three major directions or axes.
These three directions are: longitudinal or directional axis along a fuselage (roll axis); lateral axis along a wing span, wing tip to wing tip (pitch axis); and a vertical axis, perpendicular to the longitudinal axis (yaw axis).
In an entire flight envelope, the aircraft must maintain stability. Any change in the stability margin of one axis will affect the other two axes or directions. Generally, directional stability is provided by a vertical stabilizer and controlled by a rudder. Pitch stability is provided by a horizontal stabilizer and controlled by an elevator. The dihedral and taper of the wing, as well as magnitude of the wing tip lift coefficient provides lateral stability and is controlled by ailerons arranged at each wing tip or close to the wing tips.
The stability of an aircraft depends on the location of the centre of gravity of the aircraft with respect to the mean lift line or centre of lift of the wing. As long as the centre of gravity is in front of the centre of lift of the wing, there will be an automatic stability built into the aircraft in proportion to the distance between the centre of lift and the centre of gravity. If this distance is large, the stability margin will also be large, but because of the large stability, the control surfaces (elevator, aileron, rudder) must be made large and have large displacement in order to have the desired effect.
A well balanced aircraft control design calls for a solid feel on all control axis with a minimum of control surface area, and actuation motion involved, in order to maintain a low aerodynamic drag. The speed range of an aircraft depends strongly on the wing area, the wing span and the lift coefficient of the wing profile, as well as the configuration and the effectiveness of the controls.
In general, the lower the coefficient of lift and the smaller the wing area while the aircraft is in cruise mode (relative high speed with minimum aero drag), the faster the aircraft will be, given the horsepower available.
During a flight, the limit of low speed operation of an aircraft is related to the particular wing design with its attendant stall speed; more precisely, the stall of the wing tip. The upper speed range is limited by the shape or profile drag, along with a given wing area and wing span, and are dominant components of the aerodynamic drag produced by the aircraft at high speed.
An aircraft must be able to operate in a wide speed range, including some very low speed manoeuvres during take off and landing, due to restricted length of landing fields and the need for low kinetic energy dissipation at touchdown. The wide speed range requirement for an aircraft presents conflicting design parameters. In low speed operation the lift of an aircraft wing is greatly reduced, which must be restored by increasing the angle of attack (the angle which the wing is positioned to the local airflow). Critically, at low speeds, as well as high speeds regardless of wing profile shape or wing plan form, after a certain positive angle to the incoming airflow, the airflow will detached from the upper part of the wing, causing the wing to lose most of its lift and be in a near stalled condition.
During a stall, the lift-induced drag (which is dominant at low speed operation) will greatly increase, slowing down the aircraft and stalling the wing even further. This condition is aggravated even more when the aircraft has to turn during the slow speed flight. The inner wing, respective to a turning direction of the aircraft, in a turn, will have a slower airflow over it due to the fact that the inner wing tip is closer to the turning centre than the outer wing tip. In addition, the steeper the bank angle the lower the lift available from the inner (lower) wing which must be compensated by applying an opposite aileron (moving down to increase the lift) of the down-moving wing. This will greatly increase the chance of completely stalling the inner wing and losing lateral control during the turn.
The conventional method to reduce this effect is to lower the incident angle of the wing tips (lower angle of attack) with respect to the rest of the wing, as well as using a wider stalling range airfoil at the wing tips.
Conventionally, aerodynamic solutions have been limited by the maximum allowable angle of attack of the wing designed employed. Further to the wing stalling problem in all configurations, stalling of the wing tips is detrimental to the lateral stability of the aircraft due to the fact that the ailerons are located at the wing tips or close by, and are controlling the lateral orientation of the wing. With the wing tip in a stalled state, the wing (aircraft) will enter a spiral dive towards the inner or lower wing. Statistically, the highest incident rate in general aviation is related to stall and spin accidents during the slow speed regime of flight, specifically during the takeoff and landing manoeuvres.
Traditionally, an overall compromised solution was needed for wider speed range aircraft designs. For the price of reduced high-speed performance, the wing span and the wing area may be increased and a higher lift airfoil may be used for the wing design. This configuration will provide more lift at lower speeds and more aerodynamic drag at higher speeds.
Conventionally, to improve this situation, a reasonably small wing is utilized with trailing and/or leading edge flaps applied during the slow speed operation, in order to increase the lift available to the aircraft. The application of the flaps imparts a large negative pitching or diving moment to the wing that must be controlled by horizontal stabilizers that are sufficiently sized to provide a stabilizing force. In addition, the conventional flaps generate increase lift only to the inner 40% to 60% of the wing, leaving the outer wing and the wing tips at a low lift coefficient, reducing the lateral stability when it is needed the most.
As the aircraft turns at low speeds, the inner wing dips into the turn, producing less lift than the outer wing tip that must be compensated by a down-applied aileron to “pick up” the inner wing and to increase the lift coefficient. This may stall the inner wing leading to a spin or spiral dive at close proximity to the ground (takeoff and landing) where recovery may not be possible.
Therefore there is a need for an improved approach to the lateral stability and reliability of lift produced by an aircraft wing that is stall-spin proof during low speed manoeuvring.
The present invention addresses this need, and provides further related advantages.